Methods for scanning a LiDAR system in two dimensions

ABSTRACT

A method of three-dimensional imaging includes scanning a LiDAR system in a first direction with a first frequency and in a second direction with a second frequency that is different from the first frequency, so that a laser beam emitted by each laser source of the LiDAR system follows a Lissajous pattern. The method further includes emitting, using each laser source, a plurality of laser pulses as the LiDAR system is scanned in the first direction and the second direction; detecting, using each detector of the LiDAR system, a portion of each laser pulse of the plurality of laser pulses reflected off of one or more objects; determining, using a processor, a time of flight for each respective laser pulse from emission to detection; and acquiring a point cloud of the one or more objects based on the times of flight of the plurality of laser pulses.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/971,548, filed on May 4, 2018, which claims the benefit ofU.S. Provisional Patent Application No. 62/574,549, filed on Oct. 19,2017, entitled “Methods For Scanning And Operating Three-DimensionalSystems,” the contents of which are hereby incorporated by reference intheir entireties.

BACKGROUND OF THE INVENTION

Three-dimensional sensors can be applied in autonomous vehicles, drones,robotics, security applications, and the like. Scanning LiDAR sensorsmay achieve high angular resolutions appropriate for such applicationsat an affordable cost. However, improved scanning apparatuses andmethods are needed.

SUMMARY OF THE INVENTION

According to some embodiments of the present invention, a scanning LiDARsystem may include a fixed frame, a first platform, and a firstelectro-optic assembly. The first electro-optic assembly may include afirst laser source and a first photodetector mounted on the firstplatform. The scanning LiDAR system may further include a first flexureassembly flexibly coupling the first platform to the fixed frame, and adrive mechanism configured to scan the first platform with respect tothe fixed frame in two dimensions in a plane substantially perpendicularto an optical axis of the LiDAR system. The scanning LiDAR system mayfurther include a controller coupled to the drive mechanism. Thecontroller may be configured to cause the drive mechanism to scan thefirst platform in a first direction with a first frequency and in asecond direction with a second frequency. The second frequency issimilar but not identical to the first frequency. In some embodiments, aratio of the first frequency and the second frequency is rational. Insome other embodiments, a ratio of the first frequency and the secondfrequency is irrational.

According to some other embodiments of the present invention, aresonator structure for operating a two-dimensional scanning LiDARsystem may include a fixed frame, and a first platform for carrying afirst electro-optic assembly of the scanning LiDAR system. The firstelectro-optic assembly may include a first laser source and a firstphotodetector. The resonator structure may further include a first setof springs flexibly coupling the first platform to the fixed frame. Thefirst set of springs may be configured to be flexed in two orthogonaldirections so as to scan the first platform in the two orthogonaldirections in a plane substantially perpendicular to an optical axis ofthe scanning LiDAR system. The first set of springs may have a firstresonance frequency in a first direction of the two orthogonaldirections and a second resonance frequency in a second direction of thetwo orthogonal directions. The second resonance frequency is similar tobut different from the first resonance frequency. In some embodiments,the first set of springs includes four rod springs, each of the four rodsprings connecting a respective corner of the first platform to thefixed frame. In some embodiments, each of the four rod springs may beconnected to the first platform via a flexible member. The flexiblemember may be stiffer in the second direction than in the firstdirection. In some other embodiments, each of the first set of springsmay include a leaf spring. The leaf spring may be convoluted. In someembodiments, the resonator structure may further include a secondplatform, and a second set of springs flexibly coupling the secondplatform to the fixed frame. The second set of springs may be configuredto be flexed in the two orthogonal directions so as to scan the secondplatform in the two orthogonal directions. A direction of motion of thesecond platform may oppose a direction of motion of the first platform.In some embodiments, the second platform may carry a secondelectro-optic assembly of the scanning LiDAR system. The secondelectro-optic assembly may include a second laser source and a secondphotodetector.

According to some further embodiments of the present invention, a methodof three-dimensional imaging using a scanning LiDAR system may includescanning an electro-optic assembly of the LiDAR system in two dimensionsin a plane substantially perpendicular to an optical axis of the LiDARsystem. The electro-optic assembly may include a first laser and a firstphotodetector. The scanning the electro-optic assembly may includescanning the electro-optic assembly in a first direction with a firstfrequency, and scanning the electro-optic assembly in a second directionsubstantially orthogonal to the first direction with a second frequency.The second frequency is similar but not identical to the firstfrequency. The method may further include emitting, using the firstlaser source, a plurality of laser pulses at a plurality of positions asthe electro-optic assembly is scanned in two dimensions, detecting,using the first photodetector, a portion of each respective laser pulseof the plurality of laser pulses reflected off of one or more objects,determining, using a processor, a time of flight between emitting eachrespective laser pulse and detecting the portion of the respective laserpulse, and constructing a three-dimensional image of the one or moreobjects based on the determined times of flight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a LiDAR sensor for three-dimensionalimaging according to some embodiments of the present invention.

FIGS. 2A and 2B show a partially complete Lissajous pattern and acompleted Lissajous pattern, respectively.

FIGS. 3A and 3B illustrate schematically a flexure mechanism forscanning a LiDAR system according to some embodiments of the presentinvention.

FIGS. 4A and 4B illustrate schematically a flexure mechanism forscanning a LiDAR system according to some other embodiments of thepresent invention.

FIG. 5 illustrate schematically a flexure mechanism for scanning a LiDARsystem according to some further embodiments of the present invention.

FIG. 6 illustrates schematically a two-dimensional scanning LiDAR systemaccording to some embodiments of the present invention.

FIG. 7 illustrates schematically a two-dimensional scanning LiDAR systemaccording to some other embodiments of the present invention.

FIG. 8 illustrates schematically a two-dimensional scanning LiDAR systemaccording to some further embodiments of the present invention.

FIGS. 9 and 10 show a perspective view and a top view, respectively, ofa two-dimensional scanning LiDAR system 900 according to someembodiments of the present invention.

FIG. 11 shows a plan view of a flexure structure that may be used in theLiDAR system illustrated in FIGS. 9 and 10 according to some embodimentsof the present invention.

FIG. 12 shows a simplified flowchart illustrating a method 1200 ofthree-dimensional imaging using a scanning LiDAR system according tosome embodiments of the present invention.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention relates generally to LiDAR systems forthree-dimensional imaging. More specifically, the present inventionrelates to methods and apparatuses for scanning a LiDAR system in twodimensions. Merely by way of example, embodiments of the presentinvention provide scanning apparatuses and methods where the scanning inboth the horizontal and vertical directions are fast, and the scanningfrequencies in the two directions are similar but not identical.

FIG. 1 illustrates schematically a LiDAR sensor 100 forthree-dimensional imaging according to some embodiments of the presentinvention. The LiDAR sensor 100 includes an emitting lens 130 and areceiving lens 140, both being fixed. The LiDAR sensor 100 includes alaser source 110 a disposed substantially in a back focal plane of theemitting lens 130. The laser source 110 a is operative to emit a laserpulse 120 from a respective emission location in the back focal plane ofthe emitting lens 130. The emitting lens 130 is configured to collimateand direct the laser pulse 120 toward an object 150 located in front ofthe LiDAR sensor 100. For a given emission location of the laser source110 a, the collimated laser pulse 120′ is directed at a correspondingangle toward the object 150.

A portion 122 of the laser pulse 120 is reflected off of the object 150toward the receiving lens 140. The receiving lens 140 is configured tofocus the portion 122 of the laser pulse 120 reflected off of the object150 onto a corresponding detection location in the focal plane of thereceiving lens 140. The LiDAR sensor 100 further includes aphotodetector 160 a disposed substantially at the focal plane of thereceiving lens 140. The photodetector 160 a is configured to receive anddetect the portion 122 of the laser pulse 120 reflected off of theobject at the corresponding detection location. The correspondingdetection location of the photodetector 160 a is conjugate with therespective emission location of the laser source 110 a.

The laser pulse 120 may be of a short duration, for example, 100 nspulse width. The LiDAR sensor 100 further includes a processor 190coupled to the laser source 110 a and the photodetector 160 a. Theprocessor 190 is configured to determine a time of flight (TOF) of thelaser pulse 120 from emission to detection. Since the laser pulse 120travels at the speed of light, a distance between the LiDAR sensor 100and the object 150 may be determined based on the determined time offlight.

According to some embodiments, the laser source 110 a may be rasterscanned to a plurality of emission locations in the back focal plane ofthe emitting lens 130, and is configured to emit a plurality of laserpulses at the plurality of emission locations. Each laser pulse emittedat a respective emission location is collimated by the emitting lens 130and directed at a respective angle toward the object 150, and incidentsat a corresponding point on the surface of the object 150. Thus, as thelaser source 110 a is raster scanned within a certain area in the backfocal plane of the emitting lens 130, a corresponding object area on theobject 150 is scanned. The photodetector 160 a is raster scanned to aplurality of corresponding detection locations in the focal plane of thereceiving lens 140. The scanning of the photodetector 160 a is performedsynchronously with the scanning of the laser source 110 a, so that thephotodetector 160 a and the laser source 110 a are always conjugate witheach other at any given time.

By determining the time of flight for each laser pulse emitted at arespective emission location, the distance from the LiDAR sensor 100 toeach corresponding point on the surface of the object 150 may bedetermined. In some embodiments, the processor 190 is coupled with aposition encoder that detects the position of the laser source 110 a ateach emission location. Based on the emission location, the angle of thecollimated laser pulse 120′ may be determined. The X-Y coordinate of thecorresponding point on the surface of the object 150 may be determinedbased on the angle and the distance to the LiDAR sensor 100. Thus, athree-dimensional image of the object 150 may be constructed based onthe measured distances from the LiDAR sensor 100 to various points onthe surface of the object 150. In some embodiments, thethree-dimensional image may be represented as a point cloud, i.e., a setof X, Y, and Z coordinates of the points on the surface of the object150.

In some embodiments, the intensity of the return laser pulse is measuredand used to adjust the power of subsequent laser pulses from the sameemission point, in order to prevent saturation of the detector, improveeye-safety, or reduce overall power consumption. The power of the laserpulse may be varied by varying the duration of the laser pulse, thevoltage or current applied to the laser, or the charge stored in acapacitor used to power the laser. In the latter case, the charge storedin the capacitor may be varied by varying the charging time, chargingvoltage, or charging current to the capacitor. In some embodiments, theintensity may also be used to add another dimension to the image. Forexample, the image may contain X, Y, and Z coordinates, as well asreflectivity (or brightness).

The angular field of view (AFOV) of the LiDAR sensor 100 may beestimated based on the scanning range of the laser source 110 a and thefocal length of the emitting lens 130 as,

${{AFOV} = {2{\tan^{- 1}\left( \frac{h}{2f} \right)}}},$where h is scan range of the laser source 110 a along certain direction,and f is the focal length of the emitting lens 130. For a given scanrange h, shorter focal lengths would produce wider AFOVs. For a givenfocal length f, larger scan ranges would produce wider AFOVs. In someembodiments, the LiDAR sensor 100 may include multiple laser sourcesdisposed as an array at the back focal plane of the emitting lens 130,so that a larger total AFOV may be achieved while keeping the scan rangeof each individual laser source relatively small. Accordingly, the LiDARsensor 100 may include multiple photodetectors disposed as an array atthe focal plane of the receiving lens 140, each photodetector beingconjugate with a respective laser source. For example, the LiDAR sensor100 may include a second laser source 110 b and a second photodetector160 b, as illustrated in FIG. 1 . In other embodiments, the LiDAR sensor100 may include four laser sources and four photodetectors, or eightlaser sources and eight photodetectors. In one embodiment, the LiDARsensor 100 may include 8 laser sources arranged as a 4×2 array and 8photodetectors arranged as a 4×2 array, so that the LiDAR sensor 100 mayhave a wider AFOV in the horizontal direction than its AFOV in thevertical direction. According to various embodiments, the total AFOV ofthe LiDAR sensor 100 may range from about 5 degrees to about 15 degrees,or from about 15 degrees to about 45 degrees, or from about 45 degreesto about 90 degrees, depending on the focal length of the emitting lens,the scan range of each laser source, and the number of laser sources.

The laser source 110 a may be configured to emit laser pulses in theultraviolet, visible, or near infrared wavelength ranges. The energy ofeach laser pulse may be in the order of microjoules, which is normallyconsidered to be eye-safe for repetition rates in the KHz range. Forlaser sources operating in wavelengths greater than about 1500 nm, theenergy levels could be higher as the eye does not focus at thosewavelengths. The photodetector 160 a may comprise a silicon avalanchephotodiode, a photomultiplier, a PIN diode, or other semiconductorsensors.

The angular resolution of the LiDAR sensor 100 can be effectivelydiffraction limited, which may be estimated as,θ=1.22λ/D,where λ is the wavelength of the laser pulse, and D is the diameter ofthe lens aperture. The angular resolution may also depend on the size ofthe emission area of the laser source 110 a and aberrations of thelenses 130 and 140. According to various embodiments, the angularresolution of the LiDAR sensor 100 may range from about 1 mrad to about20 mrad (about 0.05-1.0 degrees), depending on the type of lenses.

As described above, a laser source and a photodetector in a LiDAR systemmay be raster scanned in two dimensions in a plane substantiallyperpendicular to an optical axis of the LiDAR system, in order to formthree-dimensional images of objects within a certain field of view.Traditionally, two-dimensional scanning may be achieved by using acombination of a relatively fast scan in one direction (e.g., a linescan) and a much slower scan in the orthogonal direction (e.g., a sweepor frame scan). For the convenience of description, the fast scan may bereferred herein as a horizontal scan, and the slow scan may be referredherein as a vertical scan. Such scanning methods may have certaindisadvantages when applied in autonomous vehicles. For example, thescanning frequency in the slow direction may correspond to the frequencyof encountering road bumps, which may affect the positional accuracy ofthe three-dimensional imaging by the LiDAR system.

Embodiments of the present invention provide scanning apparatuses andmethods where the scanning in both the horizontal and verticaldirections are fast, and the scanning frequencies in the two directionsare similar but not identical. The resulting trajectory of the lasersource or the photodetector may be characterized by a Lissajous pattern(also known as Lissajous curve or Lissajous figure). Mathematically, aLissajous curve is a graph of parametric equations:x=A sin(at+δ),y=B sin(bt),where a and b are the frequencies in the x direction (e.g., thehorizontal direction) and y direction (e.g., the vertical direction),respectively; t is time; and δ is a phase difference.

FIG. 2A shows a partially complete Lissajous pattern. FIG. 2B shows acompleted Lissajous pattern. The appearance of the pattern may besensitive to the ratio a/b and the phase difference δ. By choosing thefrequencies a and b in the two orthogonal directions to be similar butdistinctly different from each other, the Lissajous pattern may exhibitmany “lobes” in both directions. The Lissajous pattern may be closedonly if the ratio a/b is rational. It may be advantageous to choose boththe ratio a/b and the phase difference δ such that the trajectory of thelaser source or the photodetector may cover a field of view uniformly.

The frame rate may be related to the difference between the twofrequencies a and b. In some embodiments, the scanning frequencies a andb may be chosen based on a desired frame rate. For instance, if a framerate of 10 frames per second is desired, a frequency of 200 Hz in thehorizontal direction and 210 Hz in the vertical direction may be chosen.In this example, the Lissajous pattern may repeat exactly from frame toframe. By choosing the two frequencies a and b to be significantlygreater than the frame rate and properly selecting the phase differenceδ, a relatively uniform and dense coverage of the field of view may beachieved.

In some other embodiments, if it is desired for the Lissajous patternnot to repeat, a different frequency ratio or an irrational frequencyratio may be chosen. For example, the scanning frequencies in the twodirections a and b may be chosen to be 200 Hz and 210.1 Hz,respectively. In this example, if the frame rate is 10 frames persecond, the Lissajous pattern may not repeat from frame to frame. Asanother example, the scanning frequencies a and b may be chosen to be201 Hz and 211 Hz, respectively, so that the ratio a/b is irrational. Inthis example, the Lissajous pattern will also shift from frame to frame.In some cases, it may be desirable to have the Lissajous pattern not torepeat from frame to frame, as a trajectory of the laser source or thephotodetector from a subsequent frame may fill in gaps of a trajectoryfrom an earlier frame, thereby effectively have a denser coverage of thefield of view.

In some embodiments, a frequency separation that is multiples of adesired frame rate may also be used. For example, the scanningfrequencies in the two directions a and b may be chosen to be 200 Hz and220 Hz, respectively. In this case, for example, a frame of either 10 Hzor 20 Hz may be used.

Two-dimensional scanning of a LiDAR system as described above may beimplemented using flexures that can be flexed in two orthogonaldirections. FIGS. 3A and 3B illustrate schematically a flexure mechanismfor scanning a LiDAR system according to some embodiments of the presentinvention. An outer frame 310 may be attached to two fixed mountingpoints 302 a and 302 b via a first set of four leaf springs 312 a-312 d.The mounting points 302 a and 302 b may be attached to a fixed frame andare fixed in space. The outer frame 310 may carry an electro-opticassembly of the LiDAR system, which may include one or more lasersources and one or more photodetectors, as described above in relationto FIG. 1 .

Each of the first set of leaf springs 312 a-312 d may be flexed left orright and up or down, so as to translate the outer frame 310 (andtherefore the electro-optic assembly carried by the outer frame 310)horizontally and vertically with respect to the fixed mounting points302 a and 302 b. For example, FIG. 3A shows that the outer frame 310 istranslated to the left relative to the fixed mounting points 302 a and302 b (as indicated by the arrow), while FIG. 3B shows that the outerframe 310 is translated downward relative to the fixed mounting points302 a and 302 b (as indicated by the arrow). In some embodiments, eachof the first set of four leaf springs 312 a-312 d may be convoluted, asillustrated in FIGS. 3A and 3B for a compact configuration.

In some embodiments, an inner frame 320 may be attached to the two fixedmounting points 302 a and 302 b via a second set of four leaf springs322 a-322 d, as illustrated in FIGS. 3A and 3B. Similar to the first setof four leaf springs 312 a-312 d, each of the second set of leaf springs322 a-322 d may be flexed left or right and up or down, so as totranslate the inner frame 320 horizontally and vertically with respectto the fixed mounting points 302 a and 302 b.

In practice, to raster scan the electro-optic assembly of the LiDARsystem horizontally and vertically, the outer frame 310 and the innerframe 320 may be vibrated at or near their resonance frequencies. Byproperly selecting the shape of the leaf springs 312 a-312 d and 322a-322 d, slightly different resonance frequencies may be achieved in thehorizontal and vertical directions. The outer frame 310 and the innerframe 320 may move in opposite directions, i.e., 180 degrees out ofphase, similar to what the two prongs of a tuning fork would do. If theweight of the outer frame 310 and the weight of the inner frame 320 areproperly balanced, their opposing motions may cancel vibrations thatwould otherwise be transmitted to the external mounts. In addition tominimizing vibration, this may also increase the resonant quality factorQ of the system, thus reducing power requirements.

In some embodiments, the inner frame 320 may carry a counterweight.Alternatively, the inner frame 320 may carry the electro-optic assemblyof the LiDAR system, and the outer frame 310 may carry a counterweight.In some other embodiments, the inner frame 320 may carry a secondelectro-optic assembly of the LiDAR system that includes one or morelaser sources and one or more photodetectors. In some furtherembodiments, the inner frame 320 may carry magnets or coils of a voicecoil motor (VCM) that provides the mechanical drive for flexing thesprings 312 a-312 d and 322 a-322 d.

FIGS. 4A and 4B illustrate schematically a resonator structure forscanning a LiDAR system according to some other embodiments of thepresent invention. A frame 410 may be attached to a pair of flexures 420a and 420 b on either side thereof. The frame 410 may carry anelectro-optic assembly of the LiDAR system. For clarity, acounter-balance frame and a set of associated flexures are not shown inFIGS. 4A and 4B.

Each of the pair of flexures 420 a and 420 b may be fabricated bycutting a plate of spring material. A convolution configuration, asillustrated in FIGS. 4A and 4B, may be used to increase the effectivelength of the spring member. One end of each of the pair of flexures 420a and 420 b may be attached to fixed mounting points 430 a-430 d. Thepair of flexures 420 a and 420 b may be flexed in both the horizontaldirection and the vertical direction, so as to translate the frame 410horizontally and vertically, as indicated by the arrows in FIGS. 4A and4B, respectively. In practice, to raster scan the electro-optic assemblyof the LiDAR system horizontally and vertically, the frame 410 may bevibrated at or near its resonance frequencies in both horizontal andvertical directions.

FIG. 5 illustrates schematically a resonator structure for scanning aLiDAR system according to some further embodiments of the presentinvention. A frame 510 may be attached to a fixed base 520 by a set offour rod springs 530 a-530 d. The frame 510 may carry an electro-opticassembly of the LiDAR system. For clarity, a counter-balance frame and aset of associated rod springs are not shown in FIG. 5 .

The rod springs 530 a-530 d may be made of spring steel such as musicwires. The rod springs 530 a-530 d may be made to have slightlydifferent resonance frequencies in the horizontal and verticaldirections. In some embodiments, this may be achieved by making the rodsprings 530 a-530 d stiffer in the horizontal direction than in thevertical direction, or vice versa. In some other embodiments, this maybe achieved by making the rod springs 530 a-530 d having a rectangularor an oval cross-section over a portion or an entire length thereof.Using springs with an oval cross-section may reduce stresses at thecorners as compared to springs with a rectangular cross-section.Alternatively, each rod spring 530 a-530 d may have a rectangularcross-section with rounded corners to reduce stress. In some furtherembodiments, the frame 510 may include features such as the grooves 540Aand 540B so that the mounting is stiffer in one direction than theother, thus inducing a difference in the resonance frequencies even ifthe rods are symmetrical in cross-section. Such mounting features mayalternatively be incorporated into the fixed base 520 as well.

Many variations of implementing the resonator structures illustrated inFIG. 4A-4B or 5 in a LiDAR system are possible. For example, the LiDARsystem may have two electro-optic assemblies, each having one or morelaser sources and one or more photodetectors. The two electro-opticassemblies may be mounted on two separate frames. Resonator structurescoupled to the two frames may be configured to move the two frames inopposite directions.

In some embodiments, voice coil motors (VCMs) may be arranged to drive asingle frame, or both frames. Natural coupling between two resonatorsmay ensure that, even if only one frame is driven, both may vibrate atapproximately equal amplitudes. The voice coil motors may have a movingcoil design or a moving magnet design. In some embodiments, the coil maybe mounted on one frame and the magnet may be mounted on the otherframe. The stiffness of a resonator for a counterweight or a VCM may beincreased along with a corresponding reduction in amplitude, such that amomentum of one frame substantially cancels the momentum of the otherframe.

According to various embodiments, separate VCMs may be used for motionsalong the two orthogonal axes, or a single VCM may be used that combinesthe drives for motions along both axes. In the latter case, a high Qresonance structure may be used to ensure that, although the single VCMis driven at both frequencies for the two axes, the frame primarilymoves at its respective resonance frequency in each respectivedirection. Piezoelectric transducers or other linear actuators may alsobe used instead of a VCM as the driving mechanism.

FIG. 6 illustrates schematically a two-dimensional scanning LiDAR system600 with a Lissajous scan mechanism according to some embodiments of thepresent invention. The LiDAR system 600 may include a fixed frame 610, afirst platform 620 movably attached to the fixed frame 610 via a firstset of flexures 670 a and 670 b, and a second platform 630 movablyattached to the fixed frame 610 via a second set of flexures 680 a and680 b. An emission lens 612 and a receiving lens 614 are mounted on thefixed frame 610. The LiDAR system 600 includes an electro-optic assemblythat may include one or more laser sources 640 and one or morephotodetectors 650. The one or more laser sources 640 and the one ormore photodetectors 650 are mounted on the first platform such that theemission surfaces of the one or more laser sources 640 lie substantiallyin a focal plane of the emission lens 612, and the detection surfaces ofthe one or more photodetectors 650 lie substantially in a focal plane ofthe receiving lens 614.

The first set of flexures 670 a and 670 b may be configured to move thefirst platform 630 left or right and in or out of the page relative tothe fixed frame 610. A voice coil motor (VCM) that comprises a pair ofcoils 660 a and 660 b and a magnet 662 may be mounted between the firstplatform 620 and the second platform 630. In some embodiments, themagnet 662 may be mounted the first platform 620, and the pair of coils660 a and 660 b may be mounted on the second platform 630, asillustrated in FIG. 6 . The VCM may be configured to move the firstplatform 620 left or right, and move the second platform 630 in theopposite direction. The second platform 630 may serve as a counterweightto the first platform 620, so that the momentum of the second platform630 may substantially cancel out the momentum of first platform 620. Insome other embodiments, the positioning of the pair of coils 660 a and660 b and the magnet 662 may be reversed; that is, the pair of coils 660a and 660 b may be mounted on the first platform 620, and the magnet 662may be mounted on the second platform 630. A second VCM (not shown) maybe used to move the first platform 620 and the second platform 630 in orout of the page.

FIG. 7 illustrates schematically a two-dimensional scanning LiDAR system700 with a Lissajous scan mechanism according to some other embodimentsof the present invention. The LiDAR system 700 is similar to the LiDARsystem 600 illustrated in FIG. 6 . But here, VCM is mounted between thefixed frame 610 and the second platform 630, where the pair of coils 660a and 660 b is mounted on the fixed frame, and the magnet 662 is mountedon the second platform 630. The VCM is configured to move the secondplatform 630, which may carry a counterweight, left or right. The firstplatform 620 that carries the electro-optic assembly may vibratesympathetically in the opposite direction of the second platform 630 ifthe resonant frequency of the first platform matches that of the secondplatform 630. A second set of coils and a second magnet (not shown) maybe used to move the second platform 630 in or out of the page.

FIG. 8 illustrates schematically a two-dimensional scanning LiDAR system800 with a Lissajous scan mechanism according to some furtherembodiments of the present invention. The LiDAR system 800 is similar tothe LiDAR system 700 illustrated in FIG. 7 . But here, the positioningof the pair of coils 660 a and 660 b and the magnet 662 is reversed.That is, the pair of coils 660 a and 660 b is mounted on the secondplatform 630, and the magnet 662 is mounted on the fixed frame 610.

FIGS. 9 and 10 show a perspective view and a top view, respectively, ofa two-dimensional scanning LiDAR system 900 according to someembodiments of the present invention. The LiDAR system 900 includes anemission lens 912 and a receiving lens 914 attached to a fixed base 910,a first frame 920 that may carry an electro-optic assembly of the LiDARsystem 900, and a second frame 930 that may carry a counterweight. Afirst set of flexures 970 may flexibly couple the first frame 920 to thefixed base 910 via a first set of flexible hinges 972. A second set offlexures 980 may flexibly couple the second frame 930 to the fixed base910 via a second set of flexible hinges 982. Each of the first set offlexible hinges 972 and the second set of flexible hinges 982 may be inthe form of a ribbon so that it may be stiffer in the Y direction (e.g.,vertical direction) than in the X direction (e.g., horizontaldirection).

FIG. 11 shows a plan view of a flexure structure that may be used in theLiDAR system 900 illustrated in FIGS. 9 and 10 according to someembodiments of the present invention. As illustrated, the flexurestructure includes a fixed base 910. The fixed base 910 may include oneor more mounting holes 912 for connecting to a fixed outer frame (notshown in FIGS. 9 and 10 ). The flexure structure may further include aset of first flexures 970. One end of each first flexure 970 may beconnected to the fixed base 910, while the other end of each firstflexure 970 may have a mounting hole 972 for connecting to the firstframe 920 that carries the electro-optic assembly of the LiDAR system900, as illustrated in FIGS. 9 and 10 . The flexure structure furtherincludes a set of second flexures 980. One end of each second flexure980 may be connected to the fixed base 910, while the other end of eachsecond flexure 980 may have a mounting hole 982 for connecting to thesecond frame 930 that carries a counterweight.

FIG. 12 shows a simplified flowchart illustrating a method 1200 ofthree-dimensional imaging using a scanning LiDAR system according tosome embodiments of the present invention. The method 1200 may include,at 1202, scanning an electro-optic assembly of the LiDAR system in twodimensions in a plane substantially perpendicular to an optical axis ofthe LiDAR system. The electro-optic assembly may include a first laserand a first photodetector. The scanning the electro-optic assembly mayinclude scanning the electro-optic assembly in a first direction with afirst frequency, and scanning the electro-optic assembly in a seconddirection substantially orthogonal to the first direction with a secondfrequency. The second frequency is similar but not identical to thefirst frequency.

The method 1200 may further include, at 1204, emitting, using the firstlaser source, a plurality of laser pulses at a plurality of positions asthe electro-optic assembly is scanned in two dimensions; and at 1206,detecting, using the first photodetector, a portion of each respectivelaser pulse of the plurality of laser pulses reflected off of one ormore objects. The method 1200 may further include, at 1208, determining,using a processor, a time of flight between emitting each respectivelaser pulse and detecting the portion of the respective laser pulse; andat 1210, constructing a three-dimensional image of the one or moreobjects based on the determined times of flight.

It should be appreciated that the specific steps illustrated in FIG. 12provide a particular method of three-dimensional imaging using ascanning LiDAR system according to some embodiments of the presentinvention. Other sequences of steps may also be performed according toalternative embodiments. For example, alternative embodiments of thepresent invention may perform the steps outlined above in a differentorder. Moreover, the individual steps illustrated in FIG. 12 may includemultiple sub-steps that may be performed in various sequences asappropriate to the individual step. Furthermore, additional steps may beadded or removed depending on the particular applications. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A method of three-dimensional imaging using aLiDAR system, the method comprising: scanning the LiDAR system in afirst direction with a first frequency and in a second directionorthogonal to the first direction with a second frequency, the LiDARsystem including an electro-optic assembly that comprises one or morelaser sources and one or more detectors, each respective detectorcorresponding a respective laser source, wherein the scanning of theLiDAR system in the first direction and the second directions scans alaser beam emitted by each respective laser source across a respectivesub-field of view, and wherein the second frequency differs from thefirst frequency such that a trajectory of each laser source follows aLissajous pattern; translating the electro-optic assembly in the firstdirection and the second direction in a plane that is substantiallyperpendicular to an optical axis of the LiDAR system; emitting, usingeach of the one or more laser sources, a plurality of laser pulses asthe LiDAR system is scanned in the first direction and the seconddirection; detecting, using each of the one or more detectors, a portionof each respective laser pulse of the plurality of laser pulsesreflected off of one or more objects; determining, using a processor, atime of flight for each respective laser pulse from emission todetection; and acquiring a point cloud of the one or more objects basedon the times of flight of the plurality of laser pulses from each lasersource.
 2. The method of claim 1 further comprising outputting the pointcloud at a frame rate that is equal to a difference between the secondfrequency and the first frequency, so that that the trajectory of eachlaser source completes a full Lissajous scan pattern in each frame. 3.The method of claim 1 further comprising outputting the point cloud at aframe rate that is equal to one half of a difference between the secondfrequency and the first frequency, so that that the trajectory of eachlaser source completes two full Lissajous scan patterns in each frame.4. The method of claim 1 further comprising outputting the point cloudat a frame rate that is equal to a fraction of a difference between thesecond frequency and the first frequency, so that that the trajectory ofeach laser source completes an integer number of full Lissajous scanpatterns in each frame, wherein the integer number is greater than two.5. The method of claim 1 wherein: the electro-optic assembly is flexiblycoupled to a fixed frame via a flexure assembly; and translating theelectro-optic assembly is performed via the flexure assembly.
 6. Themethod of claim 5 wherein: the flexure assembly has a first resonantfrequency in the first direction, and a second resonant frequency in thesecond direction; and the first frequency is substantially equal to thefirst resonant frequency, and the second frequency is substantiallyequal to the second resonant frequency.